In recent years, there has been a growing trend to incorporate advanced various functions into electronic devices, such as mobile phones and car navigation systems. In such a trend, a touch panel has been widely employed for ease of operation. For example, a light-transmissive touch panel is attached on the front surface of a liquid crystal display (LCD) element. A user presses a touch panel with a finger or stylus, while viewing letters, symbols and graphics shown by the LCD element on the back surface of the touch panel and selects a desired function. As the demand for a touch panel increases, manufacturers have sought a user-friendly touch panel with good visibility.
A conventional touch panel will be described with reference to FIG. 4.
FIG. 4 is a sectional view of a conventional touch panel. For sake of clarity, the figure shows dimensions enlarged in the thickness direction.
The touch panel shown in FIG. 4 has upper substrate 1, lower substrate 2, upper conductive layer 3 and lower conductive layer 4. Substrates 1 and 2 are made of a light-transmissive film. Conductive layers 3 and 4 are also formed of light-transmissive material, such as indium-tin oxide. Upper conductive layer 3 is disposed on the bottom surface of upper substrate 1, and lower conductive layer 4 is disposed on the top surface of lower substrate 2; conductive layers 3 and 4 are sandwiched between upper substrate 1 and lower substrate 2.
A plurality of dot spacers (not shown) are formed with insulating resin at predetermined spaced intervals on the top surface of lower conductive layer 4. A pair of upper electrodes (not shown) is formed at both ends of upper conductive layer 3; similarly, at both ends of lower conductive layer 4, a pair of lower electrodes (not shown) is formed in an arrangement perpendicular to that of the upper electrodes.
Frame-shaped spacer 5 is disposed between upper conductive layer 3 and lower conductive layer 4 to separate them. Spacer 5 has an adhesive layer (not shown) coat-formed on the top and bottom surfaces or either one of the surfaces, by which the peripheries of upper substrate 1 and lower substrate 2 are bonded with each other. Upper conductive 3 faces lower conductive layer 4 at predetermined spaced intervals.
Upper phase-difference plate 6 with ¼ wavelength, which is an oriented film having birefringence made of polycarbonate, is disposed on the top surface of upper substrate 1, and similarly formed lower phase-difference plate 7 is disposed on the bottom surface of lower substrate 2. The phase-difference plates above are bonded to each substrate with adhesive (not shown).
Polarizing plate 8 has a laminated structure in which polarizing layer 8a is sandwiched between upper protective layer 8b and lower protective layer 8c. For example, polarizing layer 8a is made of polyvinyl alcohol with iodine and dye oriented, while protective layers 8b, 8c are made of triacetyl cellulose. Such structured polarizing plate 8 is attached on upper phase-difference plate 6.
On the other hand, polarizing plate 9 is attached on LCD element 10. The touch panel structured above is disposed on LCD element 10 through polarizing plate 9, and each pair of upper electrodes and lower electrodes is connected to the electronic circuit (not shown) of the electronic device.
Now will be described how such structured touch panel works. A user presses the touch panel with a finger or stylus, viewing the displays shown by LCD element 10 on the back surface of the touch panel. The application of depressing force depresses down a position of polarizing plate 8, upper phase-difference plate 6 and upper substrate 1, allowing a position of upper conductive layer 3 corresponding to the depressed position to make contact with lower conductive layer 4. Through the connection of the conductive layers, voltage is applied to the upper electrodes and the lower electrodes from the electric circuit. The electronic circuit detects the depressed position from voltage ratio between the electrodes. The electronic device thus recognizes the user's selection and switches to the desired function.
In the operation above, extraneous light 11 such as sunlight and lamplight comes from above and passes through polarizing plate 8. Here, suppose that polarizing plate 8 absorbs Y-directional light wave perpendicular to X-direction light wave. In this case, extraneous light 11 changes to X-directional linearly polarized light 11a and enters into upper phase-difference plate 6 from polarizing plate 8. This light, as a result of passing through upper phase-difference plate 6 with ¼ wavelength, changes from linearly polarized light to circularly polarized light and reflects upward (as reflected light 11b) at lower conductive layer 4.
When reentering upper phase-difference plate 6, reflected light 11b undergoes another ¼ polarization, so that reflected light 11b goes into polarizing plate 8 as a Y-directional linearly polarized light with a phase-shift of ½ wavelength. Polarizing plate 8 allows only X-directional light wave to pass through. Reflected light 11b, which is a Y-directional light wave, is blocked by polarizing plate 8. The extraneous light 11 entered the touch panel from above reflects upward at lower conductive layer 4, however, the reflected light 11b is blocked by polarizing plate 8 and does not exit from polarizing plate 8 that serves as an operation panel. The structure offers a reflection-free good visibility of LCD element 10 on the back surface of the touch panel.
For example, Japanese Patent Unexamined Publication No. 2006-79149 is known in a prior-art reference relating to the present invention.
According to the conventional touch panel, polarizing plate 8 is a laminated structure in which polyvinyl-alcohol-made polarizing layer 8a is sandwiched between upper and lower protective layers 8b, 8c made of triacetyl cellulose. Triacetyl cellulose is a relatively low heat-resistant material, up to approx. 90° C. When the structure is subjected to great heat, for example, in full sunlight, warpage or distortion can occur. Besides, humidity can be a factor of deteriorating polarizing characteristics. The aforementioned inconveniencies can cause mechanical distortion of the touch panel and deteriorate optical characteristics, resulting in poor visibility of LCD element 10 disposed on the back surface of the touch panel.